Jurors deciding whether Colorado movie massacre gunman James Holmes should be executed began the punishment phase of his trial on Wednesday, considering aggravating factors, including his murder of a six-year-old girl.

"She had four separate gunshot wounds to her little body," prosecutor Rich Orman told the jury, referring to Veronica Moser-Sullivan, the youngest victim of the 12 Holmes killed. Seventy people were wounded.

The jury of nine women and three men last week rejected the 27-year-old California native's plea of insanity, finding him guilty on all 165 counts related to his July 20, 2012, rampage at a midnight premiere of a Batman film.

The penalty phase of the former neuroscience graduate student's trial is expected to last about a month. For Holmes to be put to death for the attack in the Denver suburb of Aurora, jurors must vote unanimously for execution. Otherwise, he will serve a life sentence with no possibility of parole.

‘Moral decision’

The jury can consider aspects including all the evidence presented since the trial began in late April; the crime's impact on victims; Holmes' character and background; and alleged aggravating and mitigating factors.

"In the penalty phase, jurors can make a moral decision rather than a factual determination," said veteran Denver defense lawyer David Lane. Lane, an opponent of the death penalty, predicted Holmes' life will be spared due to evidence of his mental illness heard in court.

During the trial, two court-appointed psychiatrists testified that, while he is severely mentally ill, the gunman was legally sane when he plotted and carried out the attack.

On Wednesday, the prosecution presented five aggravating factors, including that Holmes' killings were committed in "an especially heinous, cruel or depraved manner," and also "while lying in wait or from ambush." Some factors that the defense could present in mitigation include the defendant's lack of any previous criminal record, his emotional state at the time, and inability to appreciate the "wrongfulness" of his conduct.

Mitigating factors also can include cooperation with law enforcement. Holmes confessed to police at the scene that he was the shooter, and he later told authorities how he rigged his apartment with explosives. The jurors will then deliberate on whether the mitigating factors outweigh the aggravating ones. If they vote unanimously that they do, the defendant gets an automatic life sentence. If not, the jury will hear victim impact testimony, and then ultimately deliberate on Holmes' fate.